Move MPS/MPO docs from ITensors.jl

[mtfishman]

Followup to #118.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.26%. Comparing base (16241c9) to head (51274ea).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   90.11%   90.26%   +0.15%     
==========================================
  Files          54       54              
  Lines        3581     3575       -6     
==========================================
  Hits         3227     3227              
+ Misses        354      348       -6     

Flag	Coverage Δ
docs	18.37% <ø> (+18.37%)	⬆️

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[emstoudenmire]

Looks good. I think it's appropriate to move all of this content here, and we can think later about things like the SiteType or OpSum system and moving some of these docs to a different place in the future for some of those systems.

[mtfishman]

Looks good. I think it's appropriate to move all of this content here, and we can think later about things like the SiteType or OpSum system and moving some of these docs to a different place in the future for some of those systems.

Yes, that's the most controversial part of this. As part of the rewrite those will be in separate packages anyway, so we can primarily document them as part of that package and then link to and maybe repeat some of that documentation in packages like ITensorMPS.jl and ITensorNetworks.jl. My goal for the rewrite is to make ITensors.jl itself much more minimal and split off more functionality into self-contained packages so I think it will be more obvious where to document things.